Mechanical Response of Crushed Stone Mixtures

Abstract

A study was conducted in order to see the effects on material performance from varying the gradation of aggregate base course material. Five different gradations were selected which followed along the North Carolina Department of Transportation’s guidelines for acceptable aggregate base mixtures, specification section 1010. Experimental trials were performed in order to classify the material of aggregate base course and to judge the performance differences between five gradations. Those experiments included grain size distribution tests, specific gravity testing, Atterberg limits testing, particle shape analysis, compaction testing, both standard (3 in. diameter) and large (6in. diameter) unconsolidated undrained compression testing, and resilient modulus testing. Generally it was found that aggregate base course acts as a well graded gravel or sand with clay like fines. The gradation was found to affect the performance of the aggregate slightly in that coarser gradations gave better strength and resilience values under conditions most similar to those found in the field. From research and experience during the lab testing it was concluded that the coarsest blends became too difficult to realistically work with and they also lacked the stability of the more well graded gradations. It was also found that as the amount of fines in the specimens exceeded around 8% by mass, those fines governed the behavior of the material. Several recommendations are proposed for the adoption of new standards and for conducting future testing on aggregate base course material. It was concluded that the current standard was sufficient in providing fairly uniform and desirable behavior. It is recommended that the gradation be narrowed and shifted toward the lower bound of the current NCDOT standard in order to have even more predictable material behavior as well as to have the benefit of higher strength and resiliency.